1. Field of the Invention
This invention relates to helicopter stability systems, and more particularly for improvements in inner loop stability systems which provide positional and attitude stability at low speeds, including hover, and provide attitude stability at cruise speeds.
2. Description of the Prior Art
As is known in the art, helicopter controls have become quite complex and are divided into essentially three general classes. One class of helicopter control provides force feel into the control sticks and pedals so that the pilot is given an indication of the commands being made to the aircraft, even though the normal control surface resistance is isolated from him by servo mechanisms and the like. The second type of control is typically referred to in helicopters as the "outer loop" or "trim" system. These are controls that actually move the control stick and pedals in lieu of the pilot, and are typically related to autopilot functions such as attitude, altitude, heading and airspeed hold. The third type of controls is typically referred to in helicopters as the "inner loop" system, and relates generally to stability augmentation. This system responds to short-term motion of the helicopter, sensed by rate gyros, to induce control signals to counteract such short-term motion, thereby to stabilize the aircraft in all of its coordinates. The outer loop controls are typically rate limited so that they cannot possibly cause more than a certain percentage of pilot authority per unit of time, but do have the capability of exercising the same full authority that the pilot may exercise. The inner loop controls, on the other hand, are typically not rate limited, since the purpose of these controls is to offset rapid, short-term changes in aircraft attitude or other flight parameters; but their total authority is typically limited to on the order of .+-.10% of the pilot's full authority. In other words, the inner loop can react instantaneously but can only augment the pilot or outer loop controls, whereas the outer loop must react more slowly, but can provide full command over the flight of the aircraft, when engaged.
In utilizing the outer loop, the pilot typically selects various trim positions for the different controls, such as a particular airspeed, a particular heading, a particular setting of the lateral and longitudinal positions of cyclic stick and the like and then engages the trim system which holds the aircraft's speed, attitude and heading in the positions set by the pilot.
In hover, altitude and attitude can be held by the outer loop (trim) system but position cannot be held, so that helicopters which are useful predominantly for earth-related functions (such as heavy lift helicopters used in the construction industry, transferring military equipment and the like) require a very heavy pilot work load in order to maintain position during earth-related operations, such as pick up or discharge of loads. Although the stability augmentation, inner loop system can eliminate some of the short-term variations in attitude and the like, these will not restore the aircraft to desired position nor tend to provide an adequate offset to changes in position which may be incurred as a consequence of inadvertent stick motion and/or wind gusts and the like. Naturally, any aircraft motion which tends to correct aircraft attitude will also cause a change in aircraft position with respect to earth. Thus stability augmentation and trim (autopilot, outer loop) functions are only partially assistive to the pilot during hover. Similarly, maintaining desired flight profile at low speeds presents problems similar to those described with respect to hover, hereinbefore. In essentially straight and level flight at cruise speeds (such as above 60 knots), a different but related demand on the pilot can result from gusty wind conditions which, although compensated for particularly in attitude by the stability augmentation system (inner loop), are not correctively compensated for in position by the inner loop or the outer loop in existing systems. This therefore causes a high degree of pilot work load during gusty wind conditions when the flight profile is of importance to the pilot.